Data processing systems require high speed processing for a large amount of data. Internally, within a data processor, increasing the speed of an LSI has been accomplished, for example through pipeline data processing. However, it has been difficult to increase the speed at which data is read from and written to a peripheral device, particularly an external memory unit. To increase the read/write speed for an external memory unit, the memory size and density has been increased, but the rate of increase has not been as great as that for internal data processing. Therefore, when a large amount of data is to be transferred and processed, the increased internal operating speed of the data processing system is not fully realized since the processing speed is limited by the read/write transfer speed with respect to the external memory unit. Accordingly, high speed transfer with an external memory unit is becoming an increasingly important objective to achieve.
In general, there are two types of external memory units, the fixed type, such as a unit having an internal hard disk in which the memory medium is fixed to the device and the detachable type, which includes floppy disks, DAT, CDs, memory cards and the like.
In a conventional data processing system, a serial data transfer system is employed for transferring the data bit by bit between the data processing device and the external memory unit. Accordingly, the transfer time is relatively long. To overcome this limitation, a disk array has been proposed for a fixed type external memory unit, in which high speed data transfer becomes possible by read and write of data in parallel to a plurality of memory media. Such a disk array of the fixed type external memory unit is disclosed in for example, "Nikkei Computer March 9," pp. 87 to 99, Nikkei BP, 1992. Further, related techniques are disclosed in Patent Application Laid-Open No. H4-279924 specification.